when looking at table 2a 7671 on site guide, the value stated for earth fault loop impedance using a bs 3036 fuse 45 amp with a protective conductor of 2.5to 16.0 is o.50ohms. now then at the begining of the guide it states the following Ze values TN-S system 0.8ohms TN-C-S system 0.35ohms TT system 21 ohms does this then mean that the 45 amp fuse can only be installed in TN-C-S systems due to the Ze values in the other systems are already to high without even taken in to account the circuits cpc?
this goes for all the stated values lower than 0.8ohms using other types of fuse.
You can still use a 45A fuse or MCB on any of the three systems.
The Regs. specify that in the event of an earth fault a protective device must open the circuit within a certain length of time (most commonly 0.4 sec. for general-purpose outlets and 5 sec. for fixed appliances).
If the loop impedance is not sufficiently low for the OCPD to meet this disconnect time, then you need to add earth-fault protection in the form of an RCD.
In the case of the quoted TT system at around 21 ohms, the fault current on a dead short to earth would be only a little over 11A, so the 45A fuse alone would offer no protection whatsoever.
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